Russia's Foreign Minister has criticized the United States for participating in Ukraine’s affairs “not always in a constructive way,” another sign of the heightened rhetorical tone between the two countries.

Addressing the Federation Council, the upper chamber of the Russian parliament, Wednesday, Sergei Lavrov said he hopes Washington convinces Ukrainian authorities to strictly adhere to the latest Minsk cease-fire agreement and avoid violence.

Lavrov referred to the recent talks in Sochi, Russia between U.S. Secretary of State John Kerry and Russian President Vladimir Putin.

Lavrov also condemned what he called radical forces in Kyiv and other capitals and warned that any provocation to “further escalate” the conflict would have “terrible consequences both for Ukraine and the entire European security system.”

Ukrainian President Petro Poroshenko said in a BBC interview that Ukraine is in a "real war" with Russia, not just with pro-Russian separatists.

Poroshenko warned that he believes Russia is preparing a military offensive for which Ukrainians must be ready. He said Ukraine must not give Russia "any tiny chance for provocation."

Kyiv and Moscow have openly expressed differences in negotiating with each other.

Poroshenko has said that he did not trust President Putin, but had no option but to negotiate with him.

The Kremlin has said it distrusted Poroshenko and accused Ukraine of “broken commitments.”

Russia blames the crisis in Ukraine on U.S. intervention in a region it has traditionally considered its sphere of influence.

The U.S. and the European Union have accused Russia of fueling the conflict in eastern Ukraine by backing separatists in the Donetsk region with weapons and troops.

President Putin and other Russian officials have consistently denied that Russian soldiers are in eastern Ukraine, insisting that any Russians fighting there are "volunteers."

According to the United Nations statistics, at least 6,000 people lost their lives since fighting erupted in eastern Ukraine in April 2014.

On Tuesday, NATO Secretary General Jens Stoltenberg met with Lavrov about the conflict in eastern Ukraine. Stoltenberg called for an end to Russia's military exercises close to the Ukrainian border and said Russia must be more transparent about its military activities.

Lavrov said before the meeting that he would listen to NATO's concerns.